The present invention relates to a method and apparatus for producing a substrate having a micro circuit pattern for a semiconductor device, a liquid crystal, or the like, and particularly relates to a technique for inspecting a pattern for a semiconductor device or a photomask, that is, the present invention relates to a technique for inspecting a pattern on a wafer in a way of semiconductor device producing process and a technique for performing comparison and inspection by using an electron beam.
Inspection of a semiconductor wafer will be described as an example.
A semiconductor device is produced by repeating a process of transferring, by lithographing and etching, a pattern formed in a photomask onto a semiconductor wafer. In a semiconductor device producing process, the state of lithographing, etching, or the like, generation of particles, and so on, exert a large influence on the yield of the semiconductor device. Accordingly, in order to detect occurrence of abnormality or failure in an early stage or preparatorily, conventionally, a method of inspecting a pattern on a semiconductor wafer is carried out in a way of producing process.
As for a method of inspecting a defect existing in a pattern on a semiconductor wafer, a defect inspecting apparatus in which white light is irradiated onto a semiconductor wafer so that circuit patterns of the same kind in a plurality of LSIs are compared with each other by using an optical image, has been put into practice. The outline of the inspecting method has been described in xe2x80x9cMonthly Semiconductor Worldxe2x80x9d, August issue, pp. 96-99, 1995. Further, as a inspecting method using an optical image, a method in which an optically illuminated region on a substrate is formed as an image by means of a time-delay integrating sensor so that the characteristic of the image is compared with designed characteristic inputted in advance to thereby detect a defect, has been disclosed in JP-A-3-167456 or a method in which the deterioration of an image at the time of acquisition of the image is monitored so that the deterioration of the image is corrected at the time of detection of the image to thereby perform comparison and inspection in a stabler optical image, has been disclosed in JP-B-6-58220. If a semiconductor wafer in a way of producing process was inspected by such an optical inspection method, the pattern residue or defect having a light-transmissible silicon oxide film or a photoresist material on its surface could not be detected. Further, an etching remainder or a incomplete-open failure in a micro conduct hole smaller than the resolution of an optical system could not be detected. Further, a defect generated in a wiring-pattern stepped bottom portion could not be detected.
As described above, with the advance of reduction in size of the circuit pattern and complication in shape of the circuit pattern and with the advance of diversification of the material, it has become difficult to detect a defect by using an optical image. Therefore, a method for comparing and inspecting a circuit pattern by using an electron beam image having higher resolution than that of the optical image has been proposed. When a circuit pattern is compared and inspected by means of an electron beam image, in order to obtain a practical inspection time, the image needs to be acquired at a very high speed in comparison with observation by using a scanning electron microscopy (hereinafter abbreviated to SEM). Further, it is necessary to secure resolution and an SN ratio in the image acquired at a high speed.
As a pattern comparison and inspection apparatus using an electron beam, a method in which an electron beam with an electron-beam current not smaller than 100 times (10 nA) as large as the current in the general SEM is irradiated onto an electrically conductive substrate (such as an X-ray mask, or the like) to detect any electrons among secondary electrons, reflected electrons and transmitted electrons generated therefrom and compare/inspect an image formed from a signal of the electrons to thereby automatically detect a defect is disclosed in J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991), J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992), JP-A-5-258703 and U.S. Pat. No. 5,502,306.
Further, as a method for inspecting or observing a circuit substrate having an insulating material by means of an electron beam, a method in which a stabler image is acquired by irradiation of a low-accelerated electron beam not higher than 2 keV in order to reduce the influence of charge has been disclosed in JP-A-59-155941 and xe2x80x9cElectron and Ion Beams Handbookxe2x80x9d (THE NIKKAN KOGYO SHINBUN, Ltd.), pp. 622-623. Further, a method in which ions are irradiated from the back of a semiconductor substrate has been disclosed in JP-A-2-15546 and a method in which light is irradiated onto a surface of a semiconductor substrate to thereby cancel charge of an insulating material is disclosed in JP-A-6-338280.
Further, in a large-current and low-accelerated electron beam, it is difficult to acquire a high-resolution image because of a space-charge effect. As a measure to solve this problem, a method in which a high-accelerated electron beam is retarded just before a sample so that a substantially low-accelerated electron beam is irradiated onto the sample is disclosed in JP-A-5-258703.
As a method for acquiring an electron-beam image at a high speed, a method in which an image is acquired by continuously irradiating an electron beam onto a semiconductor wafer on a sample stage while continuously moving the sample stage is disclosed in JP-A-59-160948 and JP-A-5-258703. Further, a structure constituted by a scintillator (Al-vapor deposited fluorescent material), a light guide and a photo-multiplier is used as a secondary electron detecting apparatus used conventionally in the SEM. A detecting apparatus of this type is, however, poor in frequency responsibility because light emission from the fluorescent material is detected, so that the detecting apparatus of this type is unsuitable for formation of an electron beam image at a high speed. As a detecting apparatus for detecting a high-frequency secondary electron signal to solve this problem, a detection means using a semiconductor detector is disclosed in JP-A-5-258703.
When a circuit pattern in a process for producing a micro-structure semiconductor device was detected by using the aforementioned prior art optical inspection method, it was possible to detect the residue of a silicon oxide film, a resist material, or the like, which was formed from an optically transmissible material and which was sufficiently short in the optical distance depending on the optical wavelength and refractive index used for inspection, and it was difficult to detect an etching remainder or a incomplete-open failure in a micro conduct hole which was linear so that the width of a short side thereof was not larger than the resolution of an optical system.
On the other hand, in the observation and inspection using the SEM, there are two problems as follows. One problem is that a very long time is required for inspecting a circuit pattern on the whole surface of a semiconductor wafer because the conventional method, by means of the SEM, for forming an electron-beam image needs a very long time. Accordingly, in order to obtain practical throughput in a semiconductor device producing process, or the like, it was necessary to acquire an electron-beam image at a very high speed. It was further necessary to secure the SN ratio of the electron-beam image acquired at a high speed and to keep accuracy in a predetermined value.
The other problem was that it was difficult to obtain a stable contrast image in inspection by means of an electron beam and to obtain a predetermined value of inspection accuracy in the case where the material constituting a circuit pattern as a subject to be inspected was formed from an electrically insulating material such as a resist, a silicon oxide film, or the like, or in the case where the material was formed from a mixture of an electrically insulating material and an electrically conducting material. This is because, when an electron beam is irradiated onto a matter, secondary electrons are generated from the irradiated portion of the material but the matter is charged because the irradiated current value is not equal to the secondary electron current value in the case where the subject to be inspected is an electrically insulating material. When charge occurs, efficiency in generation of secondary electrons from that charged portion and the orbit of secondary electrons after the generation are influenced so that not only the contrast of the image is changed but also the image is distorted without reflection of the actual shape of the circuit pattern. This charge state is sensitive to the condition of electron-beam irradiation, so that if the speed or range of irradiation of the electron beam is changed, an image quite different in contrast is obtained even in one and the same position and even in one and the same circuit pattern.
In order to detect a defect being unable to be detected by an optical inspection method with respect to the prior art, a method in which inspection is carried out by irradiating a narrowed electron beam onto a sample substrate at a high speed is disclosed in JP-A-59-160948 and JP-A-5-258703 as a method for performing comparison and inspection by means of an electron-beam image acquired by irradiating an electron beam onto an electrically conductive substrate. In this conventional technique, however, there is no description about a method for adjusting the inspection condition with respect to a material such as an electrically insulating material, or the like. Further, as another conventional technique, a method in which a primary electron beam to be irradiated onto a sample substrate is retarded to thereby make irradiation energy low-accelerated, for example, not higher than 2 keV, in order to observe a substrate having an electrically insulating material is described in JP-A-59-155941. This conventional technique is, however, a method in which an electron beam is continuously irradiated onto a certain local region so that an image is acquired after the charge of the local region becomes stable. Accordingly, this conventional technique is unsuitable for inspecting a wide region at a high speed because a long time is required for acquiring the electron-beam image. Further, even in the case where charge in the local region is stable, it is difficult that another region to be compared is controlled to be in the same charge state. For example, it is difficult to inspect a wide region of a semiconductor wafer, or the like.
In the case where not only a converged electron beam small in electron-beam current is slowly irradiated onto a sample but also a long time is taken for signal detection as shown in the conventional SEM, a signal detected in a detection time per unit pixel is integrated to form an image signal of the unit pixel so that an SN ratio necessary for comparison and inspection is obtained. Because the state of charge changes with the passage of time correspondingly to the irradiation time as described above, the image signal changes during integration so that it is difficult to obtain stable contrast. The present inventors have found that, as a method for inspecting a circuit substrate having such an electrically insulating material, it is effective for obtaining stable contrast to shorten the secondary electron signal detection time to thereby eliminate the contrast fluctuation due to the aforementioned process such as integration, or the like, and to thereby suppress the influence on the change of charge with the passage of time. Further, the present inventors have found that an electron-beam image of contrast due to the secondary electron generation efficiency of the material of the sample by irradiating a large probe sized electron beam in a range of from about 10 nm to about 50 nm onto a sample at a high speed to acquire an image instantaneously is more suitable than an electron-beam image based on contour information of a shape acquired by an electron beam converged to a range of from 5 nm to 10 nm as shown in the SEM of the conventional technique. As described above, a theme of the present invention is not only to acquire an electron-beam image of contrast generated from a material instantaneously by scanning a large probe sized electron beam at a high speed in comparison with the conventional technique but also to secure the SN ratio or resolution in the electron-beam image sufficiently adapted for image comparison and inspection.
In order to radiate an electron beam at a high speed, detect a signal at a high speed and secure the SN ratio and resolution in the electron-beam image as described above, an electron beam having an electron-beam current larger than that generally used in the SEM needs to be irradiated onto a substrate to be inspected as described in the prior art. As described in the prior art, with a large-current and low-accelerated electron beam, it is difficult to obtain an image of high resolution because of the space-charge effect. As a method to solve this problem, there is a method in which a high-accelerated electron beam is retarded just before a sample so that a substantially low-accelerated electron beam is irradiated onto the sample. In order to carry out the deceleration of the primary electron beam, a negative voltage for deceleration is required to be applied to a sample substrate, a sample stage, or the like. When the primary electron beam retarded by the negative voltage is irradiated onto the sample substrate, secondary electrons having energy of the order of tens of mV are generated from a surface of the substrate. Because an electric field generated by the negative voltage for deceleration acts on the secondary electrons to accelerate the secondary electrons to energy of the order of kV, it is difficult to collect the high-speed secondary electrons to a detector. As a method for collecting secondary electrons to a detector, there has been proposed, in the prior art, a method using a deflector (hereinafter referred to as ExB deflector) for offsetting the quantities of deflection caused by the electric field and magnetic field acting on the primary electron beam and for deflecting secondary electrons by superposing the quantities of deflection on each other. In the case where the detector is located in a place away from the orbit of the primary electron beam, however, the secondary electrons need to be deflected largely by the aforementioned ExB deflector in order to collect the secondary electrons to the detector. If the quantity of deflection is selected to be too large, there arises a problem that the secondary electrons collide with a deflection plate per se of the ExB deflector so that the secondary electrons cannot be led to the detector. Further, if the deflection by the ExB deflector is selected to be intensive, there arises a problem that aberration occurs in the primary electron beam so that it is difficult to converge the electron beam on a surface of the sample substrate through an objective lens, or the like.
Further, as described in the prior art, in order to form an electron-beam image at a high speed, a detection means using a semiconductor detector is used as a detection apparatus for detecting a high-frequency secondary electron signal. This prior art means comprises a semiconductor detector reversely biased and high in response speed, a preamplifier for amplifying an analog signal detected by the semiconductor detector, and means for light-transmitting the analog signal amplified by the preamplifier. The aforementioned semiconductor detector and the aforementioned preamplifier are floated to a positive high electric potential. In this conventional method, the analog signal detected by the semiconductor detector is transmitted as it is by the light-transmitting means. This light-transmitting means, however, is constituted by a light-emitting element for converting an electric signal into a light signal, an optical fiber cable, and a light-receiving element for converting a light signal into an electric signal. Accordingly, there arises a problem that noise generated from the light-emitting element and the light-receiving element is added to the original analog signal to lower the SN ratio in the secondary electron signal.
A first object of the present invention is to provide an inspection technique in which a circuit pattern which is hardly detected on the basis of an optical image and is formed from a material having electrically insulating property or a circuit pattern which is formed from a mixture of a material having electrically insulating property and a material having an electrically conducting property can be inspected by using an electron-beam image.
A second object of the present invention is to acquire an electron-beam image as a good-quality image which is high in the speed, good in the stability, high in the resolution, high in the contrast and large in the SN ratio in order to perform inspection by using the aforementioned electron-beam image so that a defect generated on a micro circuit pattern can be detected accurately in comparison and inspection.
A third object of the present invention is to provide a technique in which a large-current relatively large probe sized electron beam is irradiated onto a sample at a high speed and generated secondary electrons are detected instantaneously and efficiently so that an electron-beam image of high contrast generated from the material of the sample is formed in a condition corresponding to the material of the sample to thereby acquire a stable electron-beam image also with respect to the electrically insulating material.
A fourth object of the present invention is to provide means for efficiently detecting secondary electrons generated by irradiation of an electron beam onto a sample or for detecting high-speed high-frequency secondary electrons with a high SN ratio to thereby achieve the aforementioned first, second and third objects.
A fifth object of the present invention is to provide a technique for inspecting a circuit pattern with high accuracy to achieve the aforementioned first through fourth objects, that is, to provide an inspection method in which the inspection is applied to a semiconductor device or other micro circuit patterns so that a result of the inspection is reflected on the condition for producing the semiconductor device, or the like, to thereby bring contribution not only to improvement in reliability of the semiconductor device, or the like, but also to reduction in the level of defectiveness.
A substrate having a micro circuit pattern such as a semiconductor device or the like may be formed not only from an electrically conductive film singly but also from an electrically conductive film and an electrically insulating material. In order to achieve the aforementioned objects for irradiating an electron beam onto a circuit pattern having an electrically insulating material to thereby detect a micro defect on a micro pattern at a high speed, a method and apparatus for inspecting a circuit pattern according to the present invention will be described below.
According to the present inventors"" discussion, it has been found that contrast changes largely in accordance with time and position when a large quantity of electron beams are unevenly irradiated onto a local region of a substrate surface containing an electrically insulating material but an electron-beam image of stable contrast can be obtained even in the electrically insulating material when an electron beam having an electric potential substantially equal to that in the periphery of a region to be inspected in the substrate is evenly irradiated onto the sample in the region to be inspected so that secondary electrons generated in a very short time are detected. This is because a signal little in the fluctuation of incidence of secondary electrons with the passage of time can be acquired by detecting secondary electrons instantaneously in a very short time even in a transitional period in which the sample is charged by the irradiation of the electron beam. Further, the present inventors have found that an electron-beam image of contrast generated by the secondary electron generating efficiency in the material of the sample by irradiation of a large probe sized electron beam in a range of from about 50 nm to about 100 nm onto the sample at a high speed to acquire an image instantaneously is more suitable for detection of a defect than an electron-beam image based on contour information of a shape acquired by a converged electron beam in a range of from 5 nm to 10 nm as represented by the SEM in the prior art. The secondary electron generating efficiency varies in accordance with the material of a sub-layer which forms a circuit pattern therein, the material of a surface pattern and the thickness of the film. Accordingly, if an electron-beam image of high contrast in the pattern and the subbing layer is acquired, a defect in the pattern generated by the material or a defect in the sub-layer can be detected easily. The secondary electron generating efficiency in each material constituting the circuit pattern varies in accordance with the condition for irradiation of an electron beam. The secondary electron generating efficiency varies also in accordance with the state of charge. Therefore, if the condition of irradiation or the condition of charge is optimized in accordance with the material so that the contrast due to the surface pattern and the sub-layer material becomes high, an electron-beam image suitable for detection of each defect can be formed in accordance with the combination of materials. To this end, therefore, there are a method in which secondary electrons generated by irradiating an electron beam once are detected in a very short time to thereby form an electron-beam image, and a method in which secondary electrons generated in a state in which the contrast is increased by irradiation of an electron beam several times in accordance with the material are detected in a very short time. Further, it has been found that there are a method in which an electron-beam image is obtained by irradiating an electron beam once or several times at a high speed and detecting secondary electrons in a very short time in a period in which the transitional change of an electric potential due to the charge is little, and a method in which an electron-beam image is obtained by irradiating an electron beam or other charged-particle beam onto a sample substrate in advance in accordance with the combination of materials and detecting secondary electrons in a very short time after the state of charge is stabilized and the contrast becomes high. Because, in any method, an electron beam is irradiated evenly onto a region to be inspected in a state in which the potential of the region to be inspected in the substrate is substantially equal to the potential of the periphery of the region, that is, in a state in which the charge is even, the contrast of the acquired image becomes substantially even between different regions to be inspected. Accordingly, when electron-beam images are compared and inspected, no false detection is caused by the fluctuation of the contrast.
It has been found that a method in which a large-current electron beam is evenly and speedily irradiated onto a substrate to be inspected and a secondary electron signal corresponding to the region of beam irradiation is instantaneously detected simultaneously with the irradiation is effective to achieve the former detection method. Because an electron beam is evenly instantaneously irradiated onto all regions to be inspected at the time of inspection, the potential of the substrate due to the charge is even in that instance. Accordingly, the influence of the transitional change of charge with the passage of time can be avoided when secondary electrons are detected instantaneously in that state. Further, because the ratio of the number of electrons incident to a sample different in accordance with the material, to the number of secondary electrons emitted from the sample can be set to be substantially uniform if the energy of the electron beam irradiated onto the sample is controlled, the contrast can be stabilized and the injury of the sample circuit substrate can be avoided. That is, even in a circuit pattern having an electrically insulating material, an image stable in contrast can be formed. The energy of the electron beam irradiated onto the sample can be controlled because the degree of deceleration can be changed by applying a negative potential to the sample or sample stage to retard the primary electron beam just above the sample and controlling the applied voltage to be variable.
As means for automatically inspecting a defect generated in a surface of a circuit pattern of a semiconductor wafer, or the like, at a high speed, secondary electrons are detected in a real time of the movement of the sample stage to form an image by scanning an electron beam at a high speed in a direction perpendicular to the direction of the movement of the sample stage while continuously moving the sample stage in one direction, and the thus formed image is compared and inspected. In order to achieve this inspection method, an electron beam is irradiated once or several times at a high speed to form an image though the condition for irradiation of the electron beam varies in accordance with the material. The image quality of the electron-beam image adapted for comparison and inspection by irradiation of the electron beam once or several times can be secured by the following four means. The first means is to radiate a high-density electron beam onto a sample to thereby secure an SN ratio in a secondary electron signal as a base of an electron-beam image necessary for inspection. Inspection is carried out by using an electron beam with a current value not lower than 270 pA, preferably, not lower than 13 nA. This current value is achieved by setting the root of the number of irradiated electrons to be sufficiently larger than the SN ratio in the electron-beam image necessary for inspection. The second means is to converge the electron beam so that the diameter of the electron beam on the sample becomes sufficiently small at the time of irradiation of a large-current electron beam onto the sample to thereby secure the resolution necessary for the inspection of a micro circuit pattern. As described in the prior art, in the observation of, by means of an electron beam, a semiconductor, or the like, having an electrically insulating material, the energy of the electron beam irradiated onto the sample is preferably selected to be low-accelerated. In a large-current and low-accelerated electron beam, aberration caused by the space-charge effect occurs so that it becomes difficult to converge the electron beam on the sample. Therefore, this problem can be solved by using the same method as described above with respect to the means for controlling the energy of the electron beam irradiated onto the sample. That is, the space-charge effect can be suppressed by generating a high-accelerated electron beam from an electron source, and the diameter of the electron beam on the sample can be converged to secure required resolution by applying a negative potential to a sample or sample stage to retard the high-accelerated electron beam just before the sample and radiate an electron beam of substantially optimum low-accelerated energy onto the sample. The third means is to efficiently lead secondary electrons generated from a surface to a detector to thereby secure the SN ratio of the electron-beam image necessary for inspection. When an electron beam is irradiated onto a sample by the second means, secondary electrons generated from the sample are contrariwise accelerated by the electric field of a negative potential for decelerating the primary electron beam. The accelerated secondary electrons are deflected by the ExB deflector so as to be led to the detector. By deflecting the secondary electrons to radiate the secondary electrons onto a metal piece provided between the light path of the primary electron beam and the detector, and further leading the low-speed secondary electrons generated from the metal piece to the detector, the quantity of deflection can be reduced without any necessity of deflecting high-speed secondary electrons largely toward the detector. By this measure, the problem which occurs when high-speed secondary electrons are deflected largely, that is, the problem of the loss of secondary electrons caused by the collision of secondary electrons with the deflector, the lowering of resolution caused by the influence on the primary electron beam, and so on, can be solved. Furthermore, by using a material high in secondary electron generating efficiency as the metal piece, a larger number of secondary electrons than the number of electrons in the primary electron beam can be obtained and, consequently, the SN ratio in the image can be improved. The fourth means is to digitize an analog signal detected by a semiconductor detector which is used as a detector for forming an image with a high SN ratio at a high speed and to transmit the thus digitized signal. There is further provided means for converting the digitized signal into a light signal, transmitting the light signal by means of an optical fiber, or the like, and converting the transmitted signal into an electric signal again. The constituent elements from the semiconductor detector to the photo-conversion means are floated at a positive potential. Thus, the analog signal detected by the semiconductor detector is subjected to light transmission after it is digitized by an AD converter. By use of the semiconductor detector, the responsibility necessary for detecting secondary electrons at a high speed can be secured. By digitized signal light-transmission, noise can be suppressed because digital discrimination between 1 and 0 is never influenced by noise even in the case where more or less noise is generated in a light-emitting element or in a light-receiving element in the photoelectric conversion means. Further, by making the elements from the semiconductor detector to the photo-conversion means float at a positive potential, secondary electrons can be led into the semiconductor detector so that circuits after the photo-conversion means can be operated at the ground potential. Accordingly, secondary electrons generated from a sample by irradiation of an electron beam onto the sample can be detected at a high speed with little influence of noise.
By the aforementioned various means, a stable image can be obtained with respect to a circuit pattern having an electrically insulating material in a condition in which the fluctuation of image contrast caused by charge is suppressed, so that a high sensitivity, high speed and high SN ratio electron-beam image can be formed.
Of the two means, that is, a method which is effective for inspecting an aforementioned circuit pattern having an electrically insulating material and in which an electron beam is irradiated once or several times at a high speed so that an electron-beam image is obtained before occurrence of the change of a potential due to charge, and a method in which an electron beam or other charge particle beam is irradiated so that an electron-beam image is obtained after the state of charge is stabilized, the former has been described. Means for the latter inspection method will be described below.
To stabilize the state of charge before inspection in the latter method, there are two means. The first means is a method in which a second charge particle beam such as an electron beam, an ion beam, plasma, an electron shower, or the like, is irradiated in advance onto a substrate to be inspected, in a sub chamber, or the like, different from a chamber used for inspection so as to charge the substrate to a positive or negative potential before inspection. The second means is to radiate a second electron beam onto a substrate to be inspected, in a state in which the substrate is carried to a chamber for inspection and placed in a position for forming an inspection image. In order to radiate the second electron beam onto the region to be inspected before inspection without influence on inspection:
(1) the time of irradiation of the first electron beam for forming an inspection image and the time of irradiation of the second electron beam are shifted from each other; (2) the first electron beam for forming an inspection image is subject to raster-scanning on the substrate to be inspected whereas the second electron beam is irradiated in a period in which the first electron beam turns back in the scanning; (3) the first electron beam for forming an inspection image and the second electron beam are coaxially simultaneously irradiated onto a substrate to be inspected so that not only the diameter of the second electron beam is sufficiently larger than that of the first electron beam but also the maximum current density of the second electron beam is sufficiently smaller than the maximum current density of the first electron beam. As for the configuration thereof, one second electron beam source or a plurality of second electron beam sources may be disposed in the peripheral portion of an opening through which the first electron beam for forming an inspection image is irradiated so that not only the respective second electron beam sources operate independently but also each second electron beam is irradiated onto a region to be inspected in a substrate to be inspected before the first electron beam is irradiated.
As means for irradiating the second charge particle beam by a method other than the aforementioned methods, the charge particle beam may be an ion beam and an ion source is provided in the peripheral portion of the opening through which the first electron beam for forming an inspection image is irradiated so that the ion beam is irradiated onto the substrate to be inspected at the time of inspection of the subject to be inspected. As for the timing of irradiation, there is no problem when the time of irradiation of the first electron beam and the time of irradiation of the ion beam are staggered in the same manner as in the case where the second charge particle beam is an electron beam or when the ion beam is irradiated in a period in which the scanning of the first electron beam turns back.
By the aforementioned means, an electron-beam image for a circuit pattern having an electrically insulating material can be formed in an always stable state of charge by irradiating the first electron beam for forming an inspection image after irradiating the second charge particle beam onto a substrate to be inspected to make the state of charge in a surface of the substrate uniform. According to the electrically insulating material, the contrast between the electrically insulating material and the sub-layer may be increased when the substrate is charged. In this case, the method of irradiating the second charge particle beam according to the present invention becomes effective because sufficient contrast for comparison and inspection cannot be obtained though the fluctuation of the electron-beam image caused by charge can be suppressed when only the condition for irradiation of the first electron beam for forming an inspection image is controlled.
In the aforementioned means for forming an electron-beam image after stabilizing the state of charge in a surface of the substrate to be inspected by the second charge particle beam, the condition and means for irradiation of the first electron beam are the same as in the means for irradiating the first electron beam once or several times so that an image is formed before the change caused by charge occurs. Further, means for performing inspection at a high speed and means for making the image quality of the electron-beam image have a high SN ratio and a high resolution at the time of high-speed inspection are the same as in the aforementioned means. Further, in the means for forming an electron-beam image for inspection by the first electron beam after irradiating the second charge particle beam to stabilize charge, means for adjusting the state of charge can be used additionally by controlling the irradiation energy of the first electron beam and second charge particle beam irradiated onto the sample. In the case where a charge particle beam other than the electron beam for forming an image is irradiated simultaneously or nearly simultaneously with the irradiation of the electron beam, there is a possibility that a secondary electron signal unnecessary for forming the original image may be generated and detected. In this case, an aperture is placed on an image plane of the surface of the substrate on which forms an image of a circuit pattern of a substrate to be inspected by secondary electrons, so that only the secondary electrons generated from the region subjected to irradiation of the electron beam for forming an inspection image can be passed through the aperture and led to the secondary electron detector to thereby eliminate unnecessary secondary electrons.
Among the means for inspecting a substrate having a micro circuit pattern by an electron beam, various means for forming an electron-beam image have been described above. Means for detecting a defect generated on a circuit pattern on the basis of an electron-beam image will be described below. Secondary electrons generated from a surface of a sample by irradiating a first electron beam onto a first region of the sample is detected at a high speed and with high efficiency, so that an image signal of the electron beam from the first region of the substrate to be inspected is obtained and stored in a first storage portion. In this occasion, if necessary, a second charge particle beam is irradiated before the irradiation of the first electron beam. Similarly, an electron-beam image of a region which is a second region of the sample and which has the same circuit pattern as that of the first region is obtained. After detailed position adjustment with respect to images of the first and second regions is carried out in an image processing portion while the electron-beam image of the second region is stored in a second storage portion, a differential image is obtained by comparison between the images of the first and second regions. A pixel in which the absolute value of the brightness of the differential image is not smaller than a predetermined threshold is determined to be a candidate for a defect. In another means, an electron-beam image of a good-quality circuit pattern different from that of the substrate to be inspected is formed in a redetermined condition and stored in the first storage portion in advance. The first region of the substrate to be inspected is irradiated with a first electron beam and the secondary electrons generated from a surface of the sample is detected at a high speed and with high efficiency to thereby obtain an electron-beam image signal of the first region of the substrate to be inspected. The image is compared with the image of the good-quality circuit pattern stored in the first storage region, and a processing is performed such that the inspection image is determined as a defect when the absolute value of the brightness of the differential image is larger than a predetermined threshold. Also in this occasion, if necessary, a second charge particle beam is irradiated before the irradiation of the first electron beam. Further, because an image is formed by irradiating the first electron beam once or several times onto each of the first and second regions of the substrate to be inspected, as means for preventing the electron beam from being irradiated onto regions other than the image-forming region, a monitor for adjusting the focal position of the first electron beam is irradiated always with a beam such as white light other than the electron beam and reflected high is monitored. Alternatively, the region to be inspected in the substrate to be inspected can be prevented from being charged locally and the cause of false detection can be eliminated by means in which an electron-beam image is acquired in a region other than the region to be inspected and the region to be inspected is managed so that inspection is always carried out by the irradiation of the electron beam in the first cycle when the condition for sensitivity and the condition for irradiation of the electron beam are set in accordance with the substrate to be inspected before inspection.
By carrying out the aforementioned inspection method, it is possible to realize a method and apparatus for inspection, in which a circuit pattern on a substrate containing an electrically insulating material is inspected at a high speed with high sensitivity by an electron beam so that a defect generated on the circuit pattern can be detected automatically.
By inspecting a substrate having a circuit pattern, for example, such as a semiconductor device in a way of producing process by using this method and apparatus, a failure or defect which is generated in the shape of the pattern by a process and which could not be detected in a semiconductor device in every step by the conventional technique, can be detected in an early stage. As a result, problems which are latent in processes, production apparatus conditions, and so on, can be actualized. Accordingly, a countermeasure against the cause of failure in high-speed high-accurate processes of producing various kinds of substrates such as semiconductor devices compared with the conventional processes can be taken. Accordingly, not only a high yield, that is, a high efficiency percentage can be secured but also high accurate inspection can be made because error detection which occurs in inspection and becomes a problem is reduced.